When fed ad libitum individuals produced a clutch of 30 eggs every ... concentrations < 2000 cells ml-' the frequency of clutch production declined to one clutch ...
MARINE ECOLOGY - PROGRESS SERIES Mar. Ecol. Prog. Ser.
I
Published August 31
Rates of egg production by the copepod Calanus marshallae in the laboratory and in the sea off Oregon, USA William T. Peterson Marine Sciences Research Center, State University of New York, Stony Brook, New York 11794, USA
ABSTRACT: Egg production by adult female Calanus marshallae Frost was studied in the laboratory by making a census of the number of eggs produced each day by individuals maintained on a diet of the diatom Thalassiosira weissflogii. When fed ad libitum individuals produced a clutch of 30 eggs every 30 h on average. Egg production rates were influenced by food concentration, r a n q n g from 0 d-' when females were kept in filtered seawater up to 24 eggs d-' at food concentrations 2 3500 cells m]-'. Maximum rates of ingestion of phytoplankton cells also occurred at 3500 cells ml-l. Females maintained at food concentrations ranging from 2000 to 3500 cells ml-' produced the same number of clutches per unit time as females maintained at >3500 cells rnl-l, but produced fewer eggs per clutch. At food concentrations < 2000 cells ml-' the frequency of clutch production declined to one clutch per 4 d. Egg production declined to 0 after l d when females were starved. When starved for periods up to 10 d, an interval of 0.4x the starvation interval was required before egg production was re-initiated. Females maintained at 3500 cells ml-' had a gross efficiency of egg production of 29.0 % and a reproductive output of 6.g0/0 of their body weight per day. The latter is 3 x the CS-to-female growth coefficient. In situ egg production rates were also determined, using the egg-ratio method, and were usually food-limited. In the sea the functional response of egg production to food concentration in situ was similar to that measured in the laboratory.
INTRODUCTION
The copepod Calanus marshallae is common in the coastal plankton of the northeast Pacific Ocean, ranging from the Bering Sea south to northern California, USA. In the Bering Sea this species occurs from March through September (Smith & Vidal 1984, 1986) and off Oregon, USA, from February through October (Peterson & Miller 1977). In both systems the highest population densities occur during summer only in the waters of the middle and inner shelf (depths < 100m) where chlorophyll concentrations are high, ranging from 5 to 20pg 1-' off Oregon (Peterson et al. 1979, Small & Menzies 1981) and 1 to 10pg 1-' in the Bering Sea (Smith & Vidal 1984, 1986). Such food concentrations span the range over which ingestion and egg production rates range from nearly zero to a maximum for the closely-related and similarly-sized C. pacificus (Frost 1985). Thus if C. marshallae has similar physiological requirements then temporal and spatial variations in phytoplankton concentration could be a key factor O Inter-ResearcNPrinted in F. R. Germany
controlling its grazing and egg production rates in the sea, and therefore its potential for maintaining a large population in the waters of the inner continental shelf. The object of the study described in this paper was to define quantitative relationships between food concentration, feeding rates and rates of egg production by adult female Calanus marshallae. One goal was to sort out the effect of food concentration on clutch size and on the frequency at which individual females produce a clutch of eggs. A second was to examine relationships between egg production rate vs food concentration and feeding rate vs food concentration. From these 2 measurements the gross efficiency of egg production was calculated. Third, laboratory measurements of fecundity were compared to in situ measurements to determine if fecundity was food-limited in the sea. Fourth, since during the upwelling season this copepod is frequently subjected to periods of very low food concentration (< 1pg chlorophyll a 1-' during periods of active upwelling events) the effects of varying periods of starvation on egg production were examined. Finally,
Mar Ecol. Prog. Ser 47: 229-237, 1988
230
the hypothesis is discussed that high phytoplankton concentration is a necessary but not sufficient condition for population maintenance and population growth in the coastal upwelling zone of the west coast of the USA.
METHODS Female Calanus marshallae Frost were collected at stations 10 to 15 km offshore from Newport, Oregon, USA, by suspending a 0.5m diameter net of 220pm mesh at a depth of 10m for a few minutes while the ship was drifting. Zooplankton was placed into 101 insulated containers fllled with surface seawater and transported to the laboratory. Within 5 h of capture, females were sorted and placed into 600ml glass containers, 1 per container. When sorting care was taken to select only those females which seemed healthy: individuals which swam erratically, had broken antennules or missing furcal setae were not used in any experiments. Females were fed Thalassiosira weissflogii which was grown in f/2 media (Guillard & Ryther 1962). The copepods were held in the laboratory with the phytoplankton food for 48 h before being used in an experiment. Mortality was always low and occurred only during this acclimation period. All work was done in a constant-temperature walk-in chamber set at a temperature of 10°C, under continuous low light (10 to 30 pE m-' S-'). This temperature was selected because it is the average value observed for the upper 20m water column over the inner and middle shelf off Oregon during summer (Huyer 1977). Relationships between phytoplankton concentration and egg production rate were studied by making a census of the number of eggs produced each day by individual females over an 8 to 13 d interval. Food concentration was measured daily using a FuchsRosenthal counting chamber (during 1977) and a Model B Coulter Counter (during 1978). The food suspension was renewed daily. Ingestion rates were calculated using equations in Frost (1972).The volume of the Thalassiosira weissflogii clone was 1100 pm3 cell-' ( = 125 pg carbon cell-'; from Strathmann 1967). Relations between (1) weight of eggs produced per day vs female weight, known as 'reproductive output' sensu Clarke 1987, and (2) weight of food consumed per day vs female weight were calculated using weight data in Peterson (1986): egg = 0.75 pg dry wt. mean weight of a female = 260 pg dry wt (n = 23). The effect of starvation on egg production was investigated by holding females in glass-fiber (0.8 pm pore size) filtered seawater for periods ranging from 1 to 10 d. After being starved they were fed ad libitum concentrations of Thalassiosira weissflogii to determine the time lag between starvation period and resumption of
egg production. Ad libitum food concentrations were determined to be at concentrations of T. weissflogu 2 3500 cells ml-' (shown below in Fig. 4). Egg cannibalism was evaluated in 2 ways. First, eggs which were known to be < 8 h old were placed into 600 m1 beakers each containing 1 female which had been underfed for several days. After 24 h the number of eggs remaining were counted. Since it is possible to distinguish between newly-laid and older eggs, the experimental eggs could be differentiated from any eggs which a female might have laid over the course of the experiment. Cannibalism was also evaluated by comparing the number of eggs in clutches that were known to be less than 2.5 h old (i. e. clutches of eggs no older than the 8-cell stage) to the number of eggs in clutches older than 2.5 h. For this analysis I used clutch size data from those females that had been fed ad libitum. The hypothesis tested was based on the assumption that if well-fed females cannibalized their eggs during the experiments then clutches that were composed of 'older' eggs should contain fewer eggs on average than clutches which were recently laid. Egg production in situ was estimated using the eggratio method of Edmondson et al. (1962). Fecundity was calculated from estimates of the abundances of eggs and females in the sea, and from laboratory measurements of egg hatchlng times as a function of temperature (Peterson 1980). Samples for study of the vertical distribution and abundance of females and eggs were collected from 5 to 9 discrete depths of 1, 5, 10, 15, 20, 30, 50, 70, and 90 m (water depth permitting) at each of 5 stations 1, 3, 5, 7, and 10 n miles from shore on 9 dates along several transect Lines over the continental shelf off Oregon and Washington during July and August 1977. Samples were collected with the aid of a pump and 10 cm diameter hose (Miller & Judkins 1981). To collect zooplankton a 0.5 m diameter net of 64 pm mesh was suspended in the pump stream. Sample volumes were 2 m3. In addition, 100 m1 samples were taken from the pump stream and analysed for chlorophyll concentration following standard techniques (Strickland & Parsons 1972). Egg-ratio fecundity was calculated from egg and female abundances integrated over the upper 50 m (for eggs) and for the entire water column (for females) then averaged for the 1, 3, 5, and 7 nm stations. RESULTS Average clutch size The mean rate of egg production by ind~vidual female Calanus marshallae fed excess amounts of food (3500 to 8000 cells rnl-' of Thalassiosira weissflogh) was 23.9 eggs female-' d-' (Fig. 1). On many census
Peterson: Egg production by Calanus marshallae
23 1
The mean number of eggs clutch-' for females fed ad libitum was regressed on female body length to test the hypothesis that larger females produced more eggs than smaller females. Variations in clutch size were found to be unrelated to female body length (r = -0.0093, n = 26, = 0.0062).
Effects of food concentration on daily egg production rates
1
11
21
31
51
41
61
EGGS PER CLUTCH
Fig. 1. Calanus marshallae. Frequency distribution of the number of eggs per clutch produced by adult females fed excess amounts of Thalassiosira weissflogii. Data are from a daily census of individual females kept in separate experimental containers
days no eggs were produced indicating that females do not produce a clutch of eggs every day. A total of 217 clutches were laid in 270 female-days of observations, or an average of 0.8 clutches d-l, equivalent to 30 h between the production of a clutch. The frequency distribution of number of clutches laid each day was: none, 53; one, 209; two, 8. Mean clutch size was 29.7 eggs. There were some slight but significant seasonal differences in clutch size (based on paired t-tests). Table 1 shows that females collected in May produced a significantly greater number of eggs than females collected in January or July to September, 39.2 eggs clutch-' vs the mean of 29.7 eggs clutch-' observed in all other dates combined.
The effect of food concentration on the rate of egg production was measured on 57 individual females. Fig. 2 shows that the rate of egg production increased linearly as food concentration increased up to a maximum of 24 eggs female-' d-l at 3500 cells ml-l. Above that concentration (equivalent to 450 pg carbon 1-' or about 10 pg chlorophyll I-') egg production was constant. The relation between feeding rate and food concentration for the same 57 females is also shown in Fig. 2. Feeding rate increased linearly over cell concentrations ranging from 300 to 3500 cells ml-l. At concentrations greater than 3500 cells ml-' ingestion was constant at 10 924 cells h-' (= 1.37 pg carbon h-' or 32.8 pg carbon d-l). At this rate an average female Calanus marshallae (260 pg dry wt, or 104 pg carbon) consumes 31.5 % body weight d-'. The ratio, egg production rate:ingestion rate, is a measure of the gross efficiency of egg production. Table 2 shows that the efficiency of egg production (calculated on a gravimetnc basis) increased from 0.22 at the lowest food concentrations (300 cells ml-l) to a maximum of 0.29 at food concentrations in excess of 3000 cells ml-l. The reproductive output of females fed ad libitum was 0.069 of body weight d-' (6.9 %) as eggs. This value is 3 x greater than the CS-to-female growth rate of 2.4 % d-' and 2 . 5 ~less than the growth rate of younger copepodites, 17.6 O/O d-' (Peterson 1985).
Table 1. Calanus marshallae. Egg production by adult females fed Thalassiosira weissflogii ad libitum. Females were acclimated to laboratory conditions for 48 h before start of each experiment. Temperatures were f 1 C" of sea surface temperature on the collection date. Data shown are average number of eggs laid by several individual females ( N Q over ) a 4 to 13 d period. N, is total number of clutches of eggs laid during each experimental period. Sr;is standard error of mean Date 6-19 Jan 1977 23-30 Jun 1977 21-29 May 1978 27 Jun-9 Jul 1978 7-10 Aug 1978 7-14 Sep 1978 15-22 Aug 1978
Temperature ("C)
NQ
NC
Mean
Sn
10 10 10 10 10 10 15
5
54
24.9
4 4 7 4 7 5
26 32 51 14
27.4
1.60 2.60 1.75 1.67 2.05 1.60 2.67
34 27
39.2 31.3 32 2 27.2 25.8
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.. :.
.
.
!
I
.
.
I
I
2 000
I
I
4000
:.
I
I
I
I
6 000
PHYTOPLANKTON C O N C E N T R A T I O N
8 000
Fig. 2 Calanus marshallae. Relation of (A) egg production rate and (B) phytoplankton ingestion rate to phytoplankton concentration for females fed diatom Thalassiosira weissflogu. Regression equations for egg production (Y)as function of food concentration (X)were: Y = 14.94 + 0.0608 X,(r = 0.88) for the steep ascending line, and Y = 198.5 - 0.0012 X,(r = -0.027) for the plateau. Slope of the plateau did not differ from zero (F1,23 = 0.015). Corresponding regression equations for ingestion rate (Y)vs phytoplankton concentration (X) were: Y = 1147.7 + 2.51 X , ( r = 0.77) for the ascendmg line, and Y = 11 677.7 - 0.28 X, (r = -0.28) for the plateau. Slope of the plateau was hfferent from zero (F1,42= 4.59)
(cells rnl-l)
Table 2. Calanus marshallae. Gross efficiency of egg production calculated from egg production and ingestion rate data shown in Fig. 2. 'C' indicates carbon calculated assuming egg C = 0.4 X egg dry wt. The phytoplankton, Thalassiosira weissflogii, contained 125 pg carbon cell-' Cells ml-I
300 500 750 1000 2000 3500
Eggs d-'
Egg C d-'
Phytoplankton cells ingested h-'
Carbon ingested d-'
Gross efficiency of egg production
4.1 5.7 7.6 9.5 24.7 28.5
i .2 1.7 2.3 2.8 7.4 8.5
1901 2403 3255 3658 8678 9933
5.7 72 9.8 11.0 26.0 29 8
0.22 0.24 0.24 0.26 0.29 0.28
Effects of food concentration on the frequency of production of a clutch of eggs Fig. 3 shows that the number of clutches produced during a n 8 d interval depended upon food concentration. Females maintained at food concentrations
ranging from 0 to 500 cells ml-' laid 0 to 2 clutches during the 8 d experimental period, whereas females maintained at food concentrations ranging from 2000 to 8000 cells ml-' produced on average 6.8 clutches over an 8 d period. The number of clutches produced reached a maximum at a food concentration of 2000
Peterson: Egg production by Calanus marshallae
233
5 DAYS STARVED
2000
4000
PHYTOPLANKTON CONCENTRATION
6 000
8000
(cells ml-'1
Fig. 3. Calanus marshallae. Relation between number of clutches produced in 8 d , and phytoplankton concentration. Integers near various data points indicate number of discrete observations at that coordinate
cells ml-l, lower than the food concentration which produced the maximum number of eggs per clutch (3500 cells ml-l). This suggests that egg production may be a 2-step physiological process: one step involves the production of a clutch of eggs on a predetermined schedule and the second step, the energetics controlling the number of eggs within that clutch. At food concentrations above 2000 cells ml-' a clutch is produced every 30 h on average but the number of eggs produced per clutch will not be a maximum until food concentrations exceed 3500 cells ml-'. Below 2000 cells ml-' (250 pg carbon I-') food-limitation not only decreases the number of eggs produced per unit time but lengthens the usual 30 h time interval between the production of a clutch.
Effects of starvation on fecundity Females which were placed into filtered seawater produced eggs only during the first 24 h following capture, indicating that eggs result from material obtained from recently ingested food rather than from body reserves. Fig. 4 shows that females fed following a single day of starvation produced eggs after an additional 24 h. However, females that were starved for periods as long as 10 d and then fed, did not resume egg production until an interval 0.4 times the number
Fig. 4. Calanus marshallae. Relation between days starved (X) and time required for resumption of egg production (YJ after females were once again fed, for 14 individual females. Integers near various data points indicate number of discrete observations at that coordinate Regression equation was Y = 0.43 X + 1.00, ( r = 0.92, F = 62.2)
of days starved. For 2 experiments the rate of egg production by females fed ad libitum following a 5 d and a 6 d period of starvation did not return to normal levels until a time lag that was twice the starvation interval (Fig. 5).
Effects of cannibalism on egg production measurements In 12 experiments, females maintained at food concentrations ranging from 300 to 650 cells ml-' ate 68 % of the eggs offered over a 24 h period. However among well-fed females there was no evidence of cannibalism: the mean number of eggs in clutches that were < 2.5 h old was not different from the number of eggs in older clutches: 30.2 k 3.91 (95 % confidence interval) and 29.7 5.94, respectively.
+
In situ fecundity Egg-ratio fecundities were correlated with chlorophyll concentration (Fig. 6 ) .Over the domain 0 < X < 10 pg chlorophyll a 1-' the slope of the regression (1.96) was the same as the slope obtained for the regression of egg production rate vs phytoplankton concentration measured in the laboratory (slope = 2.00
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CHLOROPHYLL (ygL.')
5
10
Fig. 6. Calanus marshallae. Scattergram of in situ egg production rate calculated from the egg-ratio method, (Y) vs in situ chlorophyll concentration (X). Chlorophyll data are averages over upper 10 m of water column and egg data for upper 50 m of water column. Solid line: regression line for egg-rauo data, Y = 1.97 X + 1.62 ( r = 0.72, F = 6.55); dashed line: regression lines from laboratory data on egg production vs food concentrabon redrawn from Fig. 2. Slopes of laboratory and in situ rates (2.00 vs 1.97 were not different)
T I M E (days]
Fig. 5. Calanus marshallae. Egg production by females starved for ( A ) S d and ( B ) 6 d then fed ad libiturn, for (A) a single female and ( B ) 5 females kept individually in separate experimental containers. Relevant statistics are a s follows: ( A ) Y = 6 . 3 6 X - 3 9 . 3 6 , ( K = 0.82, F = 10.8, Sb = 1.93);( B ) Y = 5.10 X - 33.01, ( r = 0.62, F = 16.0, Sb = 1.27)
when the laboratory data were converted into eggs d-' a n d food concentration data expressed in terms of equivalent chlorophyll concentration, assuming C : chl = 4 0 ) . At concentrations > 1 0 lig chlorophyll a 1-' there were too few data points to decide if the overall relation was linear or curvilinear.
DISCUSSION Results show that in the laboratory a n d in the sea e g g production by Calanus marshallae is highly dependent upon food supply. When deprived of food, individuals cease to produce eggs within 24 h. The implication is that female Calanus spp. produce eggs from food ingested during the 24 h preceedlng a n egg laying event, suggesting a close coupling between food supply a n d fecundity, a result noted by others (Checkley 1980 for Paracalanus p a m s , Durbin et al. 1983 for Acartia tonsa, Frost 1985 for Calanus pacificus, Hirche
& Bohrer 1987 for C. glacialis, and Peterson & Bellantoni 1987 for Temora longicornis and C. chilensis). Because of this close coupling between egg production and food supply, fecundity of female C. marshallae in the Oregon upwelling zone is likely to be highly dependent upon short-term variations in food concentration caused by upwelling events. The expectation is that e g g production should be high during relaxation of upwelling (when chlorophyll concentrations frequently exceed 10 pg I-') and low during active upwelling events (when chlorophyll concentrations in the nearshore zone are usually less than 1 yg l k l ) . This was seen in the in situ fecundity data (Fig. 6 ) thus leading to acceptance of the hypothesis that the presence of high phytoplankton concentrations is a necessary condition for C. marshallae to establish a large population in the nearshore zone of the coastal upwelling system off the west coast of the United States. Prolonged periods of starvation (in excess of a few days) seemed to be detrimental to oogenesis of female Calanus marshallae. Individuals which were starved for several days then fed exhibited a lag of 3 to 6 d before resuming egg production. This response is similar to that reported for C. glacialis by Hirche & Bohrer ( 1 9 8 7 ) . C. marshallae living in the Oregon upwelling zone encounter periods of low food concentration during active upwelling events. Such events are charac-
Peterson: Egg production by Calanus rnarshallae
terized by low chlorophyll concentrations (
